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I have a Panasonic PIR sensor (EKMC1603111) and am measuring the voltage between the output pin and ground pin.

  • On the green multimeter I read 0v when there is no motion and 3.3v when there is motion (as expected)
  • On my new yellow multimeter I read 3.05v when there is no motion and 3.3v when there is motion.

enter image description here

The yellow multimeter has auto-range, the green multimeter does not. To prevent other factors I left the ends of the leads clamped to the PIR sensor and just plugged the leads out off one multimeter and then into the other. The results with different PIR sensors and different (3.3v and 5v) power sources were the same.

Possibly related: I started this debugging process because I connect the PIR sensor to a Wemos D1 Mini board and the GPIO input put read a constant digital HIGH. This was unexpected behavior but made more sense when I measured that the sensor only switches between 3v en 3.3v during motion. It did not make sense anymore when the green multimeter did show it switches between 0v and 3.3v during motion.

Is one of the multimeters wrong? Is it phantom voltage? Is it related to the permanent digital high reading by the ESP8266 board's digital input pin?

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    \$\begingroup\$ It looks blue to me. \$\endgroup\$
    – Andy aka
    Jul 2, 2020 at 13:05

1 Answer 1

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The sensor output is "open" when there is no movement, so it will not pull the output line actively to ground (you have to do this with a pull-down resistor, e.g. 100k to ground).

Possibly the green meter has a lower input impedance and pulls the output line low, while the yellow meter does not. Try adding the additional pull-down resistor, but keep in mind that the output current can not exceed 100µA. So something around 100 kOhm will be alright.

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  • \$\begingroup\$ Based on the specifications of the DIGI402B and other Mastech multimeters (can't find the MS8302D, which could also mean, it doesn't exist) both claim a 10 MOhm input impedance. But you can use both meters to measure the input impedance of both. \$\endgroup\$
    – Arsenal
    Jul 2, 2020 at 14:36
  • \$\begingroup\$ That would be to assume, probably. But with these el cheapo meters I'm not sure how far you can trust the specs. I would just test what effect an additional pull down will have, then we will see. \$\endgroup\$
    – jusaca
    Jul 2, 2020 at 18:20
  • \$\begingroup\$ That did the trick. Thank you. I ended up using a 10 kOhm resistor. I think I confused pull-up and pull-down because the Wemos board has these built-in pull-up resistors. This solved the issues with the constant high digital read and the reading on the multimeters. \$\endgroup\$
    – Niels
    Jul 3, 2020 at 11:23

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